1. Field of the Invention
The invention relates to a testing device for the testing of the electrical voltage of cables and cable sets used in high and medium voltage applications, especially cables with PE- or XLPE-isolation. In particular, the invention relates to a device for testing cables consisting of a rectifying electric voltage testing apparatus which, through a switch, places a charge on the cable up to a desired voltage.
2. Description of the Prior Art
In order to examine the performance and safety of cables after installation, after repair or after certain periods, cables are subjected to a test voltage which is higher than their maximum working voltage. The magnitude and duration of test voltages are specified by pertinent regulations. In regard to the core-ground-voltage of cables, it is possible, for instance, to test by either double effective alternating voltages of 50 Hz or by sixfold rectified voltages.
In general, the rectified voltage test is preferred since it can be executed with a substantially lesser effort than an alternating voltage test. A testing apparatus employing rectified voltage need only generate a spark current or a shunt current of a few millampere. On the other hand, an alternating voltage test apparatus must generate a relatively high reactive energy, depending on the cable capacity. Even though only one core of the cable is tested at a given time, reactive currents of more than 10 ampere may be needed. Due to cost, size and weight, alternating voltage test apparatus are, therefore, very expensive and thus not widely used.
In addition, the rectified voltage testing of PE-isolated cables may present a problem since field-disturbing space charges may be generated by the test. This, together with the strain due to polarity changes during a voltage spike, can cause significant cable damage. The voltage spike need not necessarily occur within the cable itself but may also occur within a coupling-box or within a final closure.
In order to avoid space charges on one hand, but also to get along with less energy on the other hand, some alternating voltage test apparatus employ testing generators which operate at a very low frequency, so-called VLF-generators. These generators work at a frequency of about 0.1 Hz. However, even with a test frequency of 0.1 Hz, the cable must be charged and discharged twice within 10 seconds. In comparison with a pure rectified voltage test with large cable capacities, this still is very expensive because considerable energy must either be stored in the test generator or transformed after each discharge into heat.